Hi Trapper,
Although the most recent leasehold arrangement was not in writing, there is nevertheless still a valid lease in place, being a contract that you can enforce at law. The difficulty with not having it in writing is the difficulty you have proving what the terms of that lease were.
In this situation where you have had the tenant for 6 years preceding the period where no written lease was in place, there is a former course of dealing between you which a court can look at and it may determine that the terms of your lease are the same as those that were formerly agreed to in writing. Also it may be that the last lease that you signed had a clause in it that said that after it expires it rolls over into a month to month lease, under the same terms.
Read your lease and see what it says about the lease continuing as a periodic lease after the lease period ends. You can certainly take the tenant to court over the debt, but I would start by sending a formal letter of demand for the funds that are owing.